Oils: A Fluid Discussion

New, Rotax 912 series engine owners in variably ask what oil should be used. There is no perfect oil and that can make this discussion an arbitrary topic. Many owners just ask their neighbor what oil to use, some call their aircraft Mfg., some consult the Rotax Suitable Operating Fluid publication SI 912-016 R2 and others think all oils are created equal. In this article we will take a look at a few oil traits and additives. It is out of this writer’s and Rotax’s scope to possibly test or comment on all the oils on the market. It is not this writer’s position to have you purchase any certain brand of oil, but only to help you understand what’s in an oil and why we need to use a good motorcycle oil in the Rotax 912 series engine for its longevity and health. I have attached an oil study comparison by an independent tester commissioned by Amsoil dated March 2006 and written by David Leitten and I want to give him credit right up front. This is a very good article and you will walk away with a much better understanding of oil additives. My article will parallel and summarize his article to make it easier to follow. His article goes a long way in having you understand why motorcycle oil is different than car oil and what additives are important to you and your Rotax 912 engine.You will need to draw your own conclusions as to the oil that you want to use, but now you will be well armed with good information. Oil tests are usually conducted under the American Society of Testing Materials (ASTM) standards. Rotax highly recommends the use of motorcycle oils and here are just a few reasons. (Note: When I use the term motorcycle it is applicable to the Rotax 912 as a reference)

The difference

Here are a few reasons we do not want to use auto oil in our Rotax 912. Our engine speeds (RPM) are much higher as is compression (10.5:1 for the 912ULS), The horse power ratio is much higher, and as also engine temperatures. The one really big difference between the auto and the Rotax 912 is that the Rotax shares its engine oil with the gearbox, where an auto engine has separate fluids for these functions. This item is a big concern when choosing motorcycle oil over auto oil, as the properties need to be very different. We do not use our aircraft as often as our cars so those extended down times cause problems such as rusting and acidity. The attached article delves more deeply into these subjects.

Oil Properties

Viscosity is a measure of an oils thickness and it helps protect the engine as oil is non compressible. If your oil is too thin it can be pressed out of the way where the metal surfaces come into contact. Some oils do not keep their viscosity rating under high work loads and shear and you could lose some of the viscosity protection. An oil that is too thick causes excessive work and temperatures for the engine and could cause some problems on a cold morning start up.

Shear protection is a measure of an oils capacity to thin out or reduce its load-carrying capacity. The Rotax 912 operates at higher rpm temperatures than an auto and the gearbox shares the engine oil. This high mechanical rpm, higher engine temperatures and close tolerance operation can cause some oils to thin out and not protect your engine as well as some other oils with higher shear properties.

ZDDP (ZincDialkyl-Dithio-Phosphate) WOW that’s a big word. This is a zinc phosphorus compound. This is a very important additive for us compared to an auto.This additive was very important in older autos as a protection against metal surfaces coming into contact. You see, for most of the last century, the almost universal method to open and close engine valves was via flat tappets (solid or hydraulic lifters if you will) (like the Rotax 912), and the ZDDP additive was there to prevent or reduce wear between the lifters and the camshaft. The ZDDP in the minute amounts of oil that will get burned and exit through the exhaust system which will shorten the life of catalytic converters. Thus the EPA mandate to eliminate ZDDP from engine oil. The auto makers have responded by designing engines that utilize roller lifters or overhead camshafts, and have no need for the protection offered by ZDDP. The older auto owners can still purchase this additive from automotive stores. This additive was reduced to around 800 ppm. It coats the metal surfaces and when they come into contact acts like a sacrificial surface. Zinc is not the most important additive here, but the zinc phosphorus combination is.

The 4 ball wear test. This is test to see if the oil can protect parts from metal to metal contact and is spelled out very well in the attached article. Zinc levels in this test seem to play a big part in an oils ability to prevent metal to metal contact.

Gear performance test is a measure of an oil’s viscosity and how additives contribute to gear protection under extreme pressure, shock, sliding and shearing forces. This is very important to the Rotax 912 gearbox. If you have ever taken a gearbox apart for an inspection you can see the gear wear from the use of certain oils and the absence of wear with good oil.

Oxidation test is a test to see how well oil holds up under heat. If the oil starts to break down from heat it will cause oxidation to occur which shortens the life of the oil and causes more carbon accumulation. Increased levels of ZDDP help prevent oxidation.

Volatility: a.k.a. Evaporation of some of the oil components causes higher oil consumption and higher viscosity. High temperature is the main cause of this problem.

Oil Acidity: Oil base stocks, but mainly their detergent additives are designed to help reduce the amount of acid build up.

Foaming: This has been an ugly word in the early days with some of our oils and it was enough of a concern amongst other reasons that Rotax, several years back (SB-912-040 R1 dated 8-2003), increased the volume of oil in our oil tank to help combat this problem. Some oils are certainly up to the task more than others. Foaming is caused because of our gearbox and engine oil combination. Foam as we know has air bubbles and this prevents the oil from protecting our metal to metal surfaces. Foaming causes increased wear and oxidation. (Note: Your oil tank dipstick should have a squared top where you grab it. The old dip stick is round.)

Rust Protection: Many of us live in humid climates and moisture in our oil and on internal parts is inevitable. This of course is one reason Rotax wants you to obtain at least 212F oil temperature to boil off any moisture. Oil does not protect or prevent rust, and inhibitors to the oil must be added. Letting an engine set for extended down times allows rust to cause pitting on metal surfaces and in our bearings. If you are unable to fly for extended periods it is better to at least start the engine and let it run at operating temperatures for a while on the ground.

To add to your reading it should be noted that the base stock oil is just as important as any additive. Without a good base stock then the additives may not help much. You should be using a full synthetic or a semi synthetic oil and not using a straight mineral based oil. The straight mineral based oil will not provide the protection from all the items we have talked about above. It lacks tenacity and robustness against everything that wants to damage the metal parts in our high performance engine. If you use auto fuel then you can use the full or semi synthetic oil and if you use 100LL use only the semi synthetic. The full synthetic with 100LL can’t suspend the lead and it tends to fall out of solution and settle in places we don’t want it to.

The bottom line

Here are a few oil stand outs and this of course is not all inclusive because not all oils around the world can be tested for our purposes. You need to make up your own mind in choosing an oil that is right for you and this is just the opinion of this writer and IRMT mechanic. These listed oils have shown good characteristics in lab test and field use. These are all motorcycle oils.

From all the research I have done, these oils contain good, high quality base stock oils and sufficient additives to support the engine.

Side note: I wanted to mention something I have heard and researched. AeroShell Sport Plus 4 was designed and is mfg. in England. Some oils are mfg. in different geographic locations and sometimes that changes their properties. So a one location mfg is a good thing. I understand that Aero Shell according to the Aero Shell Sport Plus 4 MSDS sheet has 1%-2% of ZDDP which translates to 1000ppm - 2000ppm.

I hope after reading these two articles that you will look at your Rotax 912 oil a bit more critically and evaluate whether you have a good oil or maybe make a change. But either way you now have more information to make that important decision.